一…就【yī … jiù】

一…就 says the second thing happens as soon as the first does. 一 goes before the first verb and 就 before the second, and both come after their subjects.

Two subjects

When each half has its own subject, the first subject goes before 一 and the second before 就, with a comma between the halves.

• × 一你来，我们就走 → 你一来，我们就走
• × 你一来，就我们走 → 你一来，我们就走

Every time

With a habit, 一…就 means "whenever": the second thing follows the first every time.

Don't drop 就

English needs only "as soon as". Here both halves are needed.

• × 我一到家睡觉了 → 我一到家就睡觉了

The 一 is the same character as the number, and changes tone the same way: yí before a fourth tone (一到), yì before the others (一回家).